Different number of coefficients for each integral

Different number of coefficients for each integral, to avoid overhead in tabulate_dof_map and filling of w[][] when there are many separate coefficients. Can quickly grow complicated though.

Alternative:
class *_integral
{
virtual bool needs_coefficient(uint i) const = 0;
}
This can return true for all coefficients it uses, such that when assembling a set of integrals the coefficients neither of them needs can be skipped.